Bone china is made by firing a mixture of clay, bone ash and a flux. Normally the clay is predominantly kaolin or china clay. The most commonly used flux is Cornish Stone but other naturally-occurring felspathic materials such as nepheline syenite may be used.
Bone ash such as used in the production of bone china is made by first treating animal bone with hot or boiling water under pressure so as to remove gelatine, collagen and other organic matter. In this state, the bone is said to be degelatinized. It is afterwards calcined to a temperature of about 1000.degree. C. in order to burn off the remaining organic matter. Calcined bone or bone ash is substantially (about 80 percent) calcium phosphate, with some calcium carbonate and a little fluoride.
Ceramic compositions for making bone china comprise typically 50% bone ash, 25% china clay and 25% Cornish stone. They are made into a ceramic body by intimately mixing and usually wet milling the particulate ingredients so as to produce an aqueous slurry or slip suitable for casting in plaster molds; or at lower water content a plastic clay which may be molded or shaped. After drying to evaporate the water, such shaped objects in the "green" state are then fired to a temperature in the region of 1250.degree. C. in order to produce unglazed "china biscuit" ware of low porosity and characteristic translucency. The biscuit ware is subsequently coated with a glaze slip and refired to a temperature of about 1100.degree. C. to produce glazed china. One of the principal characteristics of ware produced in this way is the translucency, coupled with whiteness, for which bone china (sometimes called fine china or English bone china) is renowned. Because of its strength, it is also possible to produce objects of thinner wall section than is the case with other forms of pottery, which enhances lightness of weight, translucency and resistance to thermal shock.
The final microstructure of bone china results from reactions during firing, the crystalline materials originally produced having been dissolved and new crystals formed. Bone china after firing consists of about 70% crystals and 30% of a glassy phase. The crystals are mainly .beta.-tricalcium phosphate and anorthite (a lime felspar). The high crystal content of bone china accounts for its good mechanical strength.
Bone china of substantially the above type and composition has been produced for some two hundred years and changes in formulation and fabrication have been evolutionary. The composition requires a higher firing temperature than is needed for earthenware and consequently the firing cycle is comparatively prolonged and the costs of both the kilns and the fuel used are higher. The firing time is also lengthened relative to earthenware in order to allow formation of the calcium phosphate-based phases which impart the characteristic strength and translucency.
Methods for making the ceramic clayware called "white ware", from clay, flint and a flux, in which certain boron-containing compositions are used as the flux are known. According to U.S. Pat. No. 3,532,522, a borate other than an alkali metal borate is used as the flux, e.g., calcium borate. According to U.S. Pat. No. 3,704,146, a ground vitreous frit which includes B.sub.2 O.sub.3 in its oxide analysis is used as the flux.